With development of information equipment, the needs for low-power and thin display apparatuses having grown, so that extensive study and development have been made on display apparatuses fitted to these needs.
Such a display apparatus is used frequently outdoors particularly as a wearable PC (personal computer) or an electronic note pad, thus being desirable that it can save power consumption and space. For this reason, e.g., such a product that a display function of a thin display such as a liquid crystal display and means for inputting coordinate data are integrated, and direct input can be effected by pressing a display item on a display surface with a stylus or finger, has been commercialized.
However, most of liquid crystal materials have no memory characteristic, so that it is necessary to continuously apply a voltage to the liquid crystal during a display period. On the other hand, a liquid crystal material having a memory function cannot readily ensure a reliability in the case of assuming its use in various environments such as outdoor environment as in the wearable PC, thus failing to be put into practical use.
In view of these circumstances, as one of thin and light display apparatuses, an electrophoretic display apparatus has been proposed (e.g., Japanese Patent Publication No. 3421494).
This type of electrophoretic display apparatus includes a pair of substrates disposed with a predetermined spacing therebetween, an insulating liquid filled in the spacing, a multiplicity of charged electrophoretic (migration) particles dispersed in the insulating liquid, a pair of electrodes disposed close to the insulating liquid, and an insulating layer disposed to cover the electrodes.
FIGS. 16(a) and 16(b) show an embodiment of a structure of such a conventional electrophoretic display device, wherein various types of display are effected by utilizing a difference in color between the case of distributing a large amount of electrophoretic particles 104 disposed in an insulating liquid 103 in a side area as shown in FIG. 16(a) and the case of collecting the electrophoretic particles 104 in a narrow area as shown in FIG. 16(b). As shown in FIGS. 16(a) and 16(b), the electrophoretic display device includes a pair of substrates 101 and 102, the insulating liquid 103, the electrophoretic particles 104, a pixel electrode 105, a common electrode 106 disposed to partition pixels, and insulating films 107 and 108 which cover the electrodes 105 and 106, respectively.
According to the inventor's analysis, in the electrophoretic display device shown in FIGS. 16(a) and 16(b), an equipotential line is indicated by dotted lines as shown in FIG. 17 when a voltage is applied between the pixel electrode 105 and the common electrode 106. As apparent from this figure, the equipotential line is dense at a pixel peripheral portion where a distance between the electrodes 105 and 106 is small, so that an electric field is strong. At a central portion of pixel G, an electric field is weak. In other words, it is found that an electric field (distribution) in a liquid layer comprising the insulating liquid 103 and the electrophoretic particles 104 is nonuniform. In such a vertical movement type electrophoretic display device that electrophoretic particles are moved between electrodes disposed on upper and lower substrates, such a nonuniform electric field is not caused to occur but in the case of a horizontal movement type electrophoretic display device as shown in FIGS. 16(a) and 16(b), the electrophoretic particles are generally moved between an electrode in pixel and an electrode at a pixel peripheral portion, so that a considerable nonuniform electric field is caused to occur.
For this reason, in the case where halftone is displayed, when the electrophoretic particles is partially moved on a display electrode depending on an applied voltage, a relationship between the applied voltage and halftone level is largely deviated from a linear relationship, so that it becomes difficult to effect control. As a result, stable gradation cannot be displayed.
Further, in the case of displaying black, by applying a negative-polarity voltage to the pixel electrode 105 to collect the electrophoretic particles on the pixel electrode, the black display is effected. However, an electric field toward the central portion of the pixel G is very weak, so that the electrophoretic particles do not reach the pixel central portion, thus failing to provide a sufficient contrast.
In a strong electric field place, the charged electrophoretic particles and counter ions cause very large polarization, so that when a short circuit is caused between the electrodes after a voltage for moving the electrophoretic particles is applied between the electrodes, a depolarization field due to repulsion between particles and attraction force between the particles and the ions is generated to move the electrophoretic particles. As a result, a display memory characteristic is lost in some cases.
On the other hand, in a color electrophoretic display device, a color filter method is most simple. In a conventional color electrophoretic display device, the color filter is formed on an opposite substrate or a reflection electrode.
In the case of forming the color filter on the opposite substrate, a cell is assembled so that the color filter and the pixel are aligned with each other. However, in this case, a positional deviation of the color filter from the pixel is caused to occur. When such a positional deviation is caused to occur, color mixture occurs between adjacent pixels, so that it is necessary to provide the opposite surface with a black matrix by a positional alignment margin in order to prevent the color mixture.
However, in the case of providing the black matrix, it is difficult to obtain a high aperture ratio. Particularly, a lowering in aperture ratio is noticeable in the case of forming high definition pixels of not more than 150 ppi or using a plastic surface having a high thermal expansion coefficient.
On the other hand, in the case of forming the color filter on the reflection electrode, as shown in FIG. 18, it is possible to suppress the lowering in aperture ratio by directly forming color filters 109a, 109b and 109c at a pixel G. In such a cell structure, a residual DC leading to burn-in is caused to occur in some cases, thus impairing a memory characteristic.
As a countermeasure thereagainst, it is possible to use a method wherein on an insulating layer, i.e., a color filter, a transparent electrode is formed to prevent a residual DC of the insulating layer to remain thereat. However, the resultant structure is complicated and in addition, light absorption by the transparent electrode cannot be negligible.
For this reason, by it has been desired that the problems of the conventional electrophoretic display device are remedied to retain the memory characteristic and improve a brightness.